The only rub I can see is that, as far as I can tell, he’s talking about making this the default for production mode. Making it the default for development mode isn’t tenable if you want to keep class reloading around, which almost everyone does. It’s just a hunch, but running without thread-safety in development seems weird when its the default in production. But, some teams run YARV in development and JRuby in production, so maybe I’m just making up things to worry about.